In computer networks, with perhaps the exception of very small ones, directory data needs to be present in more than one location to be equally useful to all users. Therefore, the directory data are replicated and the active directory service maintains several replicas of the directory data on multiple domain controllers to ensure a more uniform degree of availability and performance for all users.
On the other hand, in a computer structure, in order to achieve optimal resource utilization and higher overall performance, work is often spread between two or more resources. A dedicated program or a hardware device is usually responsible for this load balancing.